1. Field of the Invention
This invention relates to sensing devices for detection of magnetic (bubble) domains and more particularly to a sensing means using tunnel junctions for detection of magnetic bubble domains.
2. Description of the Prior Art
Many sensing techniques are known for detection of magnetic bubble domains. These detection means generally rely upon the influence on the detecting element of the stray magnetic field due to the bubble domain. For instance, a sense loop comprising a conductor is shown for detection of domains in U.S. Pat. No. 3,460,116. The change in magnetic flux linking the sense conductor while a domain passes below it provides the output signal in a known way.
Another type of bubble domain sensing is that using magneto-optic readout, as shown in U.S. Pat. No. 3,515,456. This sensing means relies upon the fact that the bubble domains have magnetization which is opposite to that of the rest of the magnetic sheet. Consequently, the polarization of an input light beam will be rotated differently when the beam passes through a bubble domain than when it passes through the rest of the magnetic sheet. This is the wellknown Kerr effect (reflection) or Faraday effect (transmission).
A four terminal sensor for magnetic bubble domains using the Hall effect is shown in U.S. Pat. No. 3,609,720. This type of sensing requires additional input leads and will not detect ultra-small domains.
The most suitable bubble domain sensing technique discovered so far is that due to magnetoresistive effects. A magnetoresistive element is located in flux coupling proximity to a magnetic domain. When the stray field of the domain intercepts the sensing element, the resistance of the element will change and this is detected as either a current or a voltage change. This type of sensing offers the advantages of easy fabrication, integration into the propagation circuitry used to move the domains, and high signal-to-noise ratios. Magnetoresistive sensing is described in more detail in an article by G. S. Almasi et al, appearing in the Journal of Applied Physics, Vol. 42, P. 1268, 1971.
As the development of magnetic bubble domain technology continues, the size of the domains is decreasing in order to increase storage density. This means that the magnetic field associated with the domains is becoming very small and detection of such domains is difficult. For instance, detection of submicron domains is a future problem which may be limiting in the design of a very high density bubble domain systems. The prior art has not addressed the detection of very small domains, except for various schemes which have been presented using magnetoresistive sensing. For instance, one such scheme involves the use of a second magnetoresistive sensor, series-connected with the first sensor, which is not in flux coupling proximity to the bubble domain. Noise compensation is achieved by this scheme in order to enhance the signal from the domain to be sensed. This noise cancellation means is shown in copending application Ser. No. 192,547 filed Oct. 26, 1971 now U.S. Pat. No. 3,736,419.
Another magnetoresistive sensing scheme designed to detect small bubble domains is that in which the uniaxial anisotropy field and shape anisotropy field are at right angles with one another, the smaller of these fields being aligned with the direction of the magnetic field from the domain. Such a detection means is described in copending application Ser. No. 193,904 filed Oct. 26, 1971 now U.S. Pat. No. 3,716,781.
In order to provide a magnetic domain sensing device having significantly increased sensitivity for detection of very small domains, applicants have discovered that tunnel junctions provide especially sensitive detectors of the stray magnetic field of the domain. This effect is enhanced when magnetic semiconductors and Schottky barrier junctions are used. Further advantages are achieved in that the structure is easily fabricated on the magnetic sheet or in close proximity to the magnetic sheet in which the domains exist, and can be provided in very dense arrays. In particular, magnetic bubble domains are easily sensed, regardless of their size.
Accordingly, it is a primary object of this invention to provide an ultra-sensitive detector of magnetic domains.
It is another object of this invention to provide a sensing device for detection of magnetic bubble domains which have sub-micron diameters.
It is still another object of this invention to provide a sensing means for detection of magnetic domains which does not rely upon a first order effect.
It is a further object of this invention to provide an ultra-sensitive detector of bubble domains which is easily fabricated as a very small detector.
It is a still further object of this invention to provide an improved sensing apparatus for detection of magnetic bubble domains which is capable of high packing densities for use in high density bubble domain systems.
It is another object of this invention to provide a sensing means for detection of magnetic bubble domains which comprises the magnetic sheet in which the domains exist.